In internal combustion engines for vehicles, the running speed or rotational speed and load of the engine are highly variable. Various operating conditions based on the combination of both variable factors are required to achieve low fuel cost and less harmful exhaust gas. This necessitates a proper control of the air-fuel ratio in the various operating conditions.
For the proper control of the air-fuel ratio, a feedback type air-fuel ratio control system is used for regulating the air-fuel ratio to obtain the optimum combustion state at all times in relation to the operating conditions A control valve is actuated in response to an output signal from an exhaust sensor for sensing the contents of the exhaust gas to control the supply of bleed air for the control of the air-fuel ratio.
An air-fuel ratio control system has been disclosed by Japanese Provisional Publication of Patent Application No. 59-103940, and uses a process for controlling starting fuel for an internal combustion engine which includes detection of the temperature of induction air in the engine after the start of the engine to determine an increment of the supplied fuel, in order to save fuel costs as well as to optimize drivability factors influenced by the engine.
With the conventional air-fuel ratio control system, control of the air-fuel ratio was achieved by switching from open control to feedback control when a water temperature sensor sensed that the water temperature of the engine had gone above a predetermined value. Open control basically refers to air-fuel ratio control which is achieved without the use of exhaust sensors or the like as feedback sensors.
In case the engine water is at a raised temperature at the start of the engine, as shown by FIG. 6, the oil temperature is also at a raised level, and so this may cause no inconvenience whatsoever. But if the engine water is at a low temperature when the engine starts, as shown in FIG. 7, the oil temperature is seen not to have risen appreciably even at the time when the engine water has reached the predetermined starting water temperature for feedback control.
This will result in a great loss of the power output of the engine, and inconveniently cause the air fuel ratio becomes lean, as shown by a broken line in FIG. 5, from about A/F 13 to about A/F 14.7 (theoretical air Fuel ratio) thereby inviting a decrease in the rotational speed of the engine and engine trouble. These problems are the result of the initiation of feedback control before the oil temperature is sufficiently increased.
Furthermore, the process for controlling the fuel at starting of the internal combustion engine as disclosed by the provisional publication of Patent Application No. 59-103940 needs a sensor for detecting the induction air temperature of the engine, which makes the structure complicated and expensive, leading to the inconvenience of economical disadvantages.
An object of the present invention is, in order to aid in eliminating the above-described inconveniences, to provide an air-fuel ratio control system which includes a water temperature sensor for detecting the engine water temperature and a control unit which is adapted to receive a detection signal from the water temperature sensor at the start of the engine so as to select a water temperature for starting feedback control. The system controls the air-fuel ratio by switching from open control to feedback control when the engine water temperature has reached the selected starting water temperature With this structure, decreases in the rotational speed of the engine and damage to the engine at the stage of transition from open control to feedback control can be avoided, and such structure need not be complicated and expensive.